The major objective of this study is to examine the growth of low-birth-weight infants fed on breast milk or on formula to determine the relationships between growth and energy balance, and between energy balance and behavioural state. The aims are to be realized by making a number of observations, viz: 1. Estimating the energy balance in healthy low-birth-weight infants over a period of two weeks after the resumption of weight gain. 2. Recording physical growth, heart rate, behavioural state and activity over the same period. The data analysis is being directed to show how growth is related to energy balance by energy intake, energy stored, energy expended and energy excreted. It is hoped that analysis of data relating to continuous heart rate, activity and state recording will improve our ability to predict total daily energy expenditure.